Atom-diatom complexes

In this section we will explain the essential mechanism of vibrational predissociation by virtue of a linear atom-diatom complex such as Ar H2. Figure 12.1 illustrates the corresponding Jacobi coordinates, t In particular, we consider the excitation from the vibrational ground state of H2 to the first excited state as illustrated in Figure 12.2. The close-coupling approach in the diabatic representation, summarized in Section 3.1, provides a convenient basis for the description of this elementary process. For simplicity of presentation we assume that the coupling between the van der Waals coordinate R and the vibrational coordinate r is so weak that it suffices to include only the two lowest vibrational states, n = 0 and n = 1, in expansion (3.4) for the total wavefunction,... 

The infrared transitions in the atom-diatom complexes will obey the following selection rules Ap = 1, and AJ = 1 or 0. Additionally, if the energy levels can be labeled with the quantum numbers j and 1, the selection rules A/ = 0 and Aj = 1 should hold approximately. 

The coordinate system needed for an atom-nonlinear molecule complex is a straightforward generalisation of that for an atom-diatom complex. The body-fixed axis system is defined as before, with Euler angles (a,, 0) specifying the orientation of R. However, it is now necessary to define an axis system (x y z) fixed in the monomer the relationship of these axes to the body-fixed axes is specified by Euler angles (0,, x)- md 0 describe the orientation of the z axis, and x describes rotations about the 2 axis. 

The dipole moments of the atom-diatomic complexes X2-Y, in contrast to the atom-atomic ones, depend also on the angle 0 between the axis of diatomic molecule and the axis passing through the atom Y and the molecule X2 (Fig. 3.2a). Note that if the nonrigidity of the diatomic molecule X2 is taken into account, then the additional dependence of dipole moment of the complex on r appears. So, in general we have a surface of the dipole moment for such complex. Nevertheless, these van der Waals complexes are relatively simple yet and they are studied intensively up to now because of their importance (see, for instance, [24—30]). 

The dipole-inducing mechanims of simple binary systems, such as atom-atom, atom-diatom, and diatom-diatom complexes with limited numbers of electrons, are well understood. For example, the exchange force-induced and dispersion-force-induced dipole components have been accurately computed for several simple systems, using quantum chemical methods (Chapter 4). The classical multipolar induction scheme is well known since the discovery of interaction-... 

We shall discuss some examples of reactions of excited van der Waals complexes. Up to now, only a few examples of a atom-diatom reactions have been studied—the atom being Hg, Ca, or Xe, and the diatom being H2 and halogen-containing molecules. These examples show clearly the new features in the reaction dynamics, such as orbital specificity, selectivity in the products, products state distribution, and observation of the intermediate states. 

Molecular astronomy of carbon molecules is very rich. Of about 120 known interstellar molecules more than three-quarters contain carbon atoms diatomic molecules include CO, CN, C2, CH, CH+, CN+, and CO+ polyatomic include CH2, CH4, C2H2j CH OH, CH3CH2OH, H2CO and HNC large complex unsaturated radicals and polycyclic aromatic hydrocarbons are also detected. These all play a role in the thermochemistry of interstellar clouds. The 2.6-millimeter line of CO diagnoses density and temperature in molecular clouds, as do other molecules. 

Here, an and a are the polarizabilities of the diatomic complex parallel and perpendicular to the internuclear separation, R12. A purely classical theory, which accounts for the electrostatic distortion of the local field by the proximity of a point dipole (the polarized collisional partner), suggests that )S(Ri2) 6ao/Ri2 with (Xq designating the permanent polarizability of an unperturbed atom. This expression is known to approximate the induced anisotropy of such diatoms fairly well. This anisotropy gives rise to the much studied pressure-induced depolarization of scattered light and to depolarized CILS spectra in general. The depolarization of light by dense systems of spherical atoms or molecules has been known as an experimental fact for a long time. It is, however, discordant with Smoluchowski s and Einstein s... 

In HYBO eoordinates the collinear atom diatom Hamiltonian has the more complex form... 

The extended geminal models have been used to calculate the interatomic potential for the ground state of diatomic complexes comprising an alkali ion and a noble gas atom NeLi+ [33] ArLi+ [34] ArNa+, NeNa+, HeNa+ [35] HeKa+ [36]. On the basis of the potentials for NeLi+ and HeKa+, mobility coefficients were calculated [37,38]. There was a very good agreement between the calculated and measured mobility coefficients. The deviation being of the order of 1% or lower. 

The present chapter mainly discusses the simplest class of atom-diatom Van der Waals molecules, the molecular hydrogen-inert gas complexes. While experimental information on the vibrational predissociation of these species is as yet relatively limited, our knowledge of the potential energy surfaces which govern their dynamics (9,10) is unequalled for any other systems. Moreover, the small reduced mass and large monomer level spacings make accurate calculations of their properties and propensities relatively inexpensive to perform. For these reasons, these species have come to be treated as prototype systems in theoretical studies of vibrational predissociation (17-25). 

Complex-Coordinate Coupled-Channel Approach to Predissociation of Atom-Diatom Van Per Waals Complexes... 

Hutson J M and Howard B J 1980 Spectroscopic properties and potential surfaces for atom-diatom Van der Waals complexes Mol. Phys. 41 1123-41... 

Interesting in this context are also theoretical studies of vibrational predissociation in van der Waals complexes by Beswick and Jortner. They conclude that the rates of vibrational predissociation of the rare gas atom-diatomic molecule complexes should be enhanced with decreasing mass of the rare-gas atom. If one could view relaxation of the guest molecule as a predissociation in a polyatomic van der W lals complex involving the guest and the nearest-neighbor rare-gas atoms, the observed trends would again be correctly predicted. 

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